The preheating provides for localized achievement of a higher temperature than the ambient conditions, from both sides of the joint and for a definite width.
Generally, the manufacturing standards regulate the requiring of the treatment; the process is then validated in the welding procedure qualification.
The basic reasons of carrying out a preheating are:
- Reducing the cooling gradient, especially below 800 °C, in order to limit the hardness values, especially in the Heat Affected Zone;
- Controlling the hydrogen diffusion coefficient, and therefore acting against the risk of cold cracking;
- Limiting the welding residual stress and the risk of cracking due to the constraint state;
- Offsetting the high thermal conduction coefficients of some alloys.